The invention relates to device fabrication. In device fabrication, insulating, semiconducting, and conducting layers are formed on a substrate. The layers are patterned to create features and spaces, forming devices, such as transistors, capacitors, and resistors. These devices are then interconnected to achieve a desired electrical function, producing an integrated circuit (IC).
Doped silicate glass is used as, for example, an insulating layer between conductive or semiconductive layers in the manufacture of ICs. In particular, the use of doped silicate glass such as borophohsosilicate glass (BPSG) is attractive due to its ability to reflow when annealed at a sufficiently high temperature. As such, doped silicate glass can be used to fill gaps relatively small gaps without voids. As the term is used herein, the term "gap" refers to any generic nonplanar feature on a given surface and may include such features as trenches or spaces between gates of transistors.
Conventionally, doped silicate glass such as BPSG is formed by various chemical vapor deposition (CVD) techniques. The BPSG is deposited at a relatively low temperature of about 400.degree. C. After deposition, the substrate is heated to a high enough temperature to cause the glass to soften. For example, annealing BPSG having B and P concentrations of about 4 wt % each at a temperature of 800.degree. C. for about 30 minutes can fill structures as narrow as 0.25 um with aspect ratios of up to 3:1 without voids.
The dopant concentration of the B in the BPSG affects its reflow or melting temperature. The higher the B concentration, the lower the reflow temperature and vice-versa. Thus, increasing the B concentration improves the glass ability to fill gaps at a given temperature. Generally, it is desirable to have as high a B concentration to enable filling of small gaps with a lower thermal budget. However, if the total dopant concentration of the BPSG or doped silicate glass exceeds an upper limit, the dopants tend to precipitate and form acid crystals on the surface. Such surface crystals adversely affect the reliability and characteristics of subsequently formed layers. Typically, the upper limit of dopant concentration is about 11 wt % (all percentages are wt %). Of course, this limit may vary depending on the type of doped silicate glass and deposition conditions.
As dimensions continue to decrease in advance IC designs, the doped silicate glass is required to fill narrower structures with higher aspect ratios. Due to the inherent upper limit in the dopant concentration of the doped silicate glass, a higher temperature anneal of longer duration is required in order to satisfy the needs of advanced IC designs. However, such an anneal typically exceeds the allowable thermal budget, resulting in a non-existing manufacturable process window.
As shown from the above discussion, it is desirable to provide filling of narrow structures with high aspect ratios having a manufacturable process window.